Forklift trucks have been used either to lift objects of relatively heavy weight up to an elevated location or to move the objects from one place to another within a limited working site. Depending on the power sources employed, the forklift trucks are classified into an engine-driven forklift truck which may usually operate in the outdoor area and an electromotive forklift truck which are suitable for indoor operation, thanks to its reduced or little emission of exhaust gas and noise. Another method of classifying the forklift trucks depends on the mobility of the mast assembly with respect to a vehicle body: a counterbalanced forklift truck whose mast assembly remains fixedly attached to the front end of the vehicle body; and a reach type forklift truck whose mast assembly is movable back and forth along reach legs with respect to the vehicle body. The counterbalanced truck is typically equipped with an internal combustion engine, while the reach type truck is provided with a counterweight and an electric motor and a battery feeding electricity to the motor, in which sense the latter is frequently referred to as an electromotive reach type forklift truck.
In case of the conventional, counterbalanced forklift truck, there is a tendency that a greater vertical load is applied to the front wheels than to the rear wheels as heavyweight cargos are loaded on the forwardly projecting forks. This means that the tire of the front wheels is crushed more heavily than the rear wheel tire, causing the forklift truck to be tilted forwardly, with the truck's center of weight shifted forwards in correspondence to the degree of slope. Such is particularly true when the cargo-loaded forklift truck moves on a downhill road. The forward shifting of the truck weight center makes the forklift truck unstable, sometimes resulting in the cargos dropped out of the forks and leading to the tumbling of the truck.
U.S. Pat. No. 5,368,322 to Korpimaa dated Nov. 29, 1994 teaches a truck spring suspension that includes hydraulic cylinders for causing a truck frame to be vertically moved between a suspended upper position and an unsuspended lower position, thus adjusting the height of the frame from the ground depending on the truck travelling condition. The truck spring suspension fails, however, to disclose or suggest an arrangement capable of returning a tilted truck frame into a horizontal position.